In a color image forming apparatus, a tandem type intermediate transfer method in which developing units configured to form toner images respectively in three primary colors (Y; yellow, M; magenta, and C; cyan) and black (Bk; black) are-arranged in sequence, the respective color toner images are overprinted at one pass on the intermediate transfer belt (primary transfer), then the overprinted color toner images are transferred in block to the paper (secondary transfer) is currently in vogue because of its superiority in high-speed printing.
In the color image forming apparatus in which the tandem-type intermediate transfer method is employed, a secondary transfer efficiency of the color toner images to the paper is not high under any circumstances as matters now stand. Therefore, there are problems such that the images may be deteriorated or uneven transfer may occur due to defect of transferred colorant or scattering of toner.
In the color image forming apparatus as such, the toner transferred from the developing unit on an upstream side to the intermediate transfer belt may be reversely transferred to the photoconductive member of the developing unit on a downstream side. In this case, there arises a problem of variations in density or hue of the image outputted onto the paper.
In addition, the image forming apparatus in a cleaner-less specification has a problem such that the reverse transfer as described above causes toner on the upstream side to be mixed with developer stored in the developing unit on the downstream side, thereby varying the hue of the toner image formed by the developing unit on the downstream side.
Improvement in the secondary transfer efficiency and restraint of the reverse transfer are mutually contradictory in benefit, and hence making the harmonized relation of these requirements clear is an important subject to study for the improvement of the quality of output images.
In contrast, a study focusing on an adhesion of the toner was conducted, and a method of deriving the adhesion of the toner quantitatively from a centrifugal force when toner particles are separated from a substance where the toner used to adhere using a centrifugal separator is proposed (for example, JP-A-2002-328484).
Also, a preferable range of the adhesion for achieving a technical element for improving the quality of the output images is reported (for example, Japanese Patent No. 3508078, JP-A-2003-270970). However, these are still in an extent such that the lower the adhesion between the belt and the toner, the better it becomes as a countermeasure for defects of a belt cleaning.
Therefore, it can be said that there is no report in which the conditions to achieve both the improvement of the secondary transfer efficiency and the restraint of the reverse transfer are studied in conjunction with the adhesion of the toner.